Mode transformer for wave guides



Jan. 29, 1957 E. M. PURCELL 2,779,923

MODE TRANSFORMER FOR WAVE GUIDES Filed May 6, 1946 INVENTOR EDWARD M. PURCELL ATTORNEY United States Patent ()fiice 2,779,923 Patented Jan. 29, 1957 MODE TRANSFORMER FOR WAVE GUIDES Edward M. Purcell, Cambridge, Mass., assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Application May 6, 1946, Serial No. 667,502

Claims. (Cl. 333-21) My present invention relates to electric wave transmis- 7 wave guide 1, the wide walls 4 of the passage 7 begin to sion systems and in particular to a mode transformer for v such a system.

In many applications of wave guide transmission lines to the carrying of electric waves, it is desirable to be able to change from a rectangular to a cylindrical wave guide or vice versa readily and simultaneously to carry waves in each type of wave guide in a preferred or desired mode. In the past this has been accomplished by an abrupt change from one type of wave guide to the other, and the transformation between preferred modes of oscillation has usually been accompanied by reflections of energy at the discontinuity of either wave guide. This results in loss of power and high standing wave ratios, which may further cause arcovers and equipment failures.

It is accordingly an object of my invention to provide a means for gradually changing a rectangular wave guide passage to a cylindrical wave guide passage or vice versa without large losses or undesirable reflections of power. Itis another object of my invention to provide such apparatus that will preserve desired modes in both types of wave guides.

It is a further object of my invention to provide such apparatus that will transform a mode of oscillation in a rectangular wave guide to another mode of oscillation in a cylindrical wave guide, or vice versa.

It is a still further object of my invention to provide such a transformer that may be cast integral with a cylindrical wave guide to provide a one-piece transformer.

These and other objects and features of my present invention will become apparent upon a careful consideration of the following detailed description when taken together with the accompanying drawing, the figures of which illustrate a typical embodiment of the invention.

Fig. 1 illustrates a transformer of my invention installed in a cylindrical wave guide;

Fig. 2 is a transverse section looking into the plane defined by the lines II-II in Fig. 1;

Fig. 3 is a transverse section of Fig. 1 looking into the plane defined by the lines III--III of Fig. 1;

Fig. 4 is a transverse section looking in the opposite direction in the same plane as Fig. 3, to wit, the plane v defined by the lines IV-IV in Fig. 1;

1 an initially rectangular wave guide passage 7 having a pair of parallel wide walls 4 and a first narrow wall 5. A segment of the inner wall 6 of the cylindrical wave guide 1 forms the second narrow wall of the passage 7. The first narrow wall 5 preferably is substantially contiguous with a diameter of the wave guide 1. In the direction away vfrom the first end 3 toward the second end 8 of the diverge about the contiguous ends of the first narrow wall 5. The manner in which the wide walls 4 diverge may be best appreciated by reference to the cross sections of Figs. 6 to 9 inclusive as compared with the cross section of Fig. 5. The walls 4 in Fig. 6 may be seen to be diverging gradually from their position in Fig. 5. Continued divergence of the walls 4 will soon form a straight line 9. When the narrow wall 5 is exactly contiguous with a diameter of the wave guide 1, the straight line 9 is a diameter of said wave guide. Simultaneously, the second narrow wall of the initially rectangular wave guide passage 7 has become an increasingly large portion of the inner wall 6 of the wave guide 1 until when the walls 4 have diverged sufliciently to form the straight line 9 the wall 6 is now semicircular. With further progress along the line 1 toward the second end 8, the wide walls 4 of the wave guide passage 7 continue to diverge in the same direction as initially, and finally to converge slowly to form a thin septum 12. Simultaneously, with this convergence of the walls 4, the first narrow wall 5 tapers substantially at a point 5'. The septum 12 is terminated by a substantially knife-edged end 10.

As will be readily appreciated by those skilled in the art, the casting 2 may be cast integral with the wall of a cylindrical wave guide 1, or may be inserted separately in such a wave guide as herein illustrated. Further, my transformer need not be cast, but may be of pressed metal or by any desired and convenient process.

The operation of my apparatus may be understood with reference to known modes of transmission in rectangular and circular wave guides. Thus if the TEo1 mode is being brought into my transformer at the rectangular wave guide end 3, the E vectors 11 will be directed substantially transverse to the passage 7, from one wide wall 4 to the other.

- Now, as the walls 4 diverge, the E vectors 11 will become increasingly longer to reach from one wide wall 4 to the other. In accordance with their nature, these E vectors 11 will be reduced substantially to zero at the arcuate wall 6 so that they follow an arcuate path. Thus the E vectors 11 gradually become circular vectors at the round wave guide end 8 of the transformer, a characteristic of the TEo1 mode for cylindrical wave guides.

My device has thus been shown to be able to change the TEo1 mode for rectangular wave guides in the TEo1 mode for cylindrical wave guides. It will be apparent that any mode having similarly directed E vectors in a rectangular or circular wave guide will be changed to a desired mode having similarly directed E vectors in the other type of wave guide by the apparatus of my invention. Therefore I prefer that my invention be not limited except as required by the prior art and the spirit of the appended claims. The apparatus illustrated herein is illustrative only and not the only manner in which my invention can be practiced.

I claim:

1. Apparatus for interchanging electric waves between a rectangular wave guide and a cylindrical wave guide comprising, means defining an elongated wave guide passage having at a first end a substantially rectangular cross section and at a second end a circular cross section, said means including at said first end first and second parallel wide walls and first and second narrow walls, said wide walls gradually diverging pivotally substantially degrees each about the contiguous ends of said first narrow wall in the direction along said passage toward said second end, said second narrow wall simultaneously becoming an increasing portion of arcuately shaped wall defined by the contiguous ends of said wide walls, said first narrow wall tapering substantially to a point toward said second end, said wide walls converging toward said second end to form a knife-edged septum, and said second narrow-wall becoming the sole wall of a cylindrical wave guide at said second end.

2. Apparatus for interchanging electric waves between a rectangular wave guide and a cylindrical waveguide comprising, an elongated cylindrical wave guide and means therein disposed defining together with said wave guide a wave guiding passage having a rectangular cross section at the first end and a circular cross section at the second end of said wave guide, said rectangular cross section being defined by first and second parallel wide walls and first and second narrow walls, said first narrow wall being at all times substantially contiguous with a diameter of said wave guide and said second narrow wall being-provided by a portion of said wave guide, said wide walls. gradually diverging pivotally substantially 180 each about the'contiguous ends ofsaid first narrow wall in thedirection along said waveguide-toward said second end, said second narrow wall simultaneously becoming an increasing portion of said wave guide wall defined by the contiguous ends ofsaid'wicle Walls, said firstnarrow wall tapering substantially to a point toward said second end, said wide wallsconverging toward said secend end to form a knife-edged septum, and said second narrow wall becoming the sole wall of said cylindrical waveguide at said second end:

3. A mode transformer for changing electromagnetic waves in a hollow wave guide from one mode to another comprising, conducting means defining'an elongated wave guide passage having a rectangular cross-section at-one end defined-by firstand second broad walls and first and second narrow Walls and a circular cross-section at the other end, said broad walls gradually diverging pivotally each about the contiguous ends of said'first narrow Wall in a direction along said passage toward said other end, and converging near said other end to form a knife-edged septum, said second narrow wall becoming an increasing portion of an arcuately shaped wall along the length of said passage and forming the sole wall of a circular wave guide at said other end.

ly diverging pivotallyeach about the contiguous ends of' said second narrow wall toward theother end of said section and converging near said other end to, form; a septum disposed along a radius of said circular wave guide, saidfirst'narrow wall simultaneously becoming an increasing arcuate portion of the-walk of said circular wave guide and becoming the sole Wall of said circular wave guide at said other end.

5. Apparatus for changing the mode of electromagnetic energy from one'in which the electric fieldvectors are linear to one in which these vectors are circular or vice versa, comprising in combination a hollow circular wave guide, a rod having a diameter equal to the internal diameter of said circular wave guide positioned therein, an electromagnetic transmission channel cut in said rod andextending longitudinally from one end thereof toapproximately its midpoint, said channel being of a U-shaped configuration at said end and'gradually changing to pie-shaped and then semkcircular, said rod gradual- 1y. changing in configuration from semi-circular atthat point Whereat said channel has this same shape to pieshaped, then rectangular, and finally tapering to a knifeedge at its other end.

References Cited in the file of this patent UNITED STATES PATENTS 2,129,714 Southworth Sept. 13, 1938 2,439,285 Clapp Apr. 6, 1.948, 2,455,158 Bradley Nov. 30, 1 948, 2,519,750 Ehlers Aug. 22, 1950 

